1) Field of the Invention
This invention relates to a drive shaft for an automotive vehicle water pump. The drive shaft is employed to rotate an impeller of the water pump for recirculating coolant for an engine of an automotive vehicle.
2) Description of the Related Art
A water pump is used to recirculate coolant through a cylinder block of a water-cooled engine in an automotive vehicle. The water pump is equipped with a drive shaft, which has a driven pulley on an end portion thereof and an impeller on an opposite end portion thereof. The impeller is positioned within a coolant passage. The drive shaft is driven by a belt mounted on a drive pulley, which is fixed on an end portion of a crankshaft of the engine, and the driven pulley. As a consequence, the impeller is rotated by the drive shaft to recirculate the coolant.
Reference is first had to FIG. 4 which is a side view of a bearing unit with a drive shaft member assembled therein. It is to be noted that a quarter of the bearing unit has been cut off to show the internal structure. Plural (two in the figure) rows of inner raceways 3,3 are formed in an outer peripheral wall of a large-diameter bearing portion 2 which is provided at an intermediate part of the drive shaft member designated at numeral 1. A like plural number (i.e., two in the figure) of outer raceways 5,5 are formed in an inner peripheral wall of an outer race 4 in radial registration with the respective inner raceways 3,3. Plural ball bearings 6,6 are arranged as rolling elements between the inner raceways 3,3 and the corresponding outer raceways 5,5, respectively, so that the drive shaft member 1 is rotatably supported inside the outer race 4. The outer race 4 is fixed on an unillustrated engine cylinder block.
At opposite end portions of the drive shaft member 1, a small-diameter pulley shaft portion 7 and a small-diameter impeller shaft portion 8 are provided in continuation with proximal end faces of the large-diameter bearing portion 2 so that the small-diameter pulley and impeller shaft portions 7,8 extend coaxially with the large-diameter bearing portion 2. A driven pulley (not shown), on which the belt driven by the drive pulley is mounted, can be fixed on the small-diameter pulley shaft portion 7 (i.e., the right-hand, small-diameter portion as viewed in FIG. 4), while an impeller (not shown) adapted to produce a flow of coolant through the coolant passage can be secured on the small-diameter impeller shaft portion 8 (i.e., the left-hand, small-diameter portion as viewed in FIG. 4).
In the drive shaft member 1, each of the small-diameter shaft portions 7,8 and the large-diameter bearing portion 2 are continuously connected via a continuously-connecting portion which presents a circular-arc outer peripheral surface as depicted in FIG. 5, so that the outer peripheral surfaces of the small-diameter shaft portions 7,8 extend in continuation with the circular-arc outer peripheral surface of the continuously-connecting portion. This continuously-connecting portion is formed in the following manner. First, lathe turning is applied by a lathe or the like to an outer peripheral wall of the drive shaft member 1, whereby a chamfered peripheral edge portion 9 and a first circular arc portion 10 are formed with their outer peripheral surfaces extending continuously from a peripheral edge of the proximal end of the large-diameter bearing portion 2.
At the time right after the formation of the first circular arc portion 10, the outer diameter of the small-diameter pulley shaft portion 7 is still greater than a desired value as indicated by a two-dot chain line a in the same figure. An outer peripheral wall of the small-diameter pulley shaft portion 7 is therefore subjected to grinding after heat treatment, so that the outer peripheral wall is removed as thick as .delta. to reduce the outer diameter of the small-diameter pulley shaft portion 7 to the desired value. By such grinding, a second circular arc portion 11 is formed at a radius of curvature, which is determined by the profile of a grinding stone employed in the grinding work, at the continuously-connecting portion between the first circular arc portion 10 and the outer peripheral surface of the small-diameter pulley shaft portion 7, whereby a connecting peripheral outer edge 12 remains as a boundary.
As a result, the large-diameter bearing portion 2 and the small-diameter pulley shaft portion 7 are continuously connected via the chamfered portion 9 and the first and second circular arc portions 10,11.
The conventional drive shaft member for an automotive vehicle water pump, in which the large-diameter bearing portion 2 and the small-diameter pulley shaft portion 7 are continuously connected in such a form as described above, may not exhibit sufficient strength in some instances. A driven pulley is fixed on the small-diameter pulley shaft portion 7 and a drive belt is mounted on the driven pulley. Substantially large tension of the belt is therefore applied to the driven pulley. In addition, a cooling fan for an engine is also mounted on the small-diameter pulley shaft portion 7. As a consequence, significant bending load is applied to the drive shaft member 1.
Under such significant bending load, substantial stress is applied to the first and second circular arc portions 10,11. There is hence the potential danger that, in the course of use of the drive shaft member 1 over a long time, a crack may occur in at least one of the circular arc portions 10,11 and the drive shaft member 1 may be broken there.
Carburizing may be applied to the surface of the drive shaft member 1 especially to improve the durability of the rolling bearing unit. When such carburizing is applied, intergranular oxide layers may be formed in black scales in the surface of the first circular arc portion 10 so that the strength of the drive shaft member 1 may be reduced considerably there. This makes the drive shaft member 1 more susceptible to breakage.